The first metal-cutting machining methods of the type stated in the introduction are known from DE 243 514 C. The gear skiving uses as the tool a toothed wheel having end-face cutters. Unlike in slotting, the cutting motion is realized in that, via a skewed arrangement of the axis of the tool and the rotational axis of the workpiece, a cutting motion is generated by oppositely directed rotations of these parts. As it circulates around the workpiece, the tool passes respectively through toothings which it cuts out of the workpiece.
In principle, a workpiece can be produced in gear skiving in a single pass with just one performed feed motion. In the case of greater material removal, however, several passes are sensible, in which the peeling tool consecutively executes two feed motions with differently large cutting depths, as is described, for instance, in DE 10 2008 037 514 A1.
In order to improve the quality of the produced workpiece, in WO 2012/098 002 A1, it is proposed that the workpiece-axis-parallel components of the feed motion and of the cutting motion are directed oppositely to each other.
WO 2010/060733 A1 relates to a gear skiving apparatus in which an electronic control device for positioning drives of the tool spindle and of the workpiece are provided, wherein the control device, in the tooth cutting of a crudely toothed or untoothed blank, in the axial feed at the end of the feed overlays a radial emergence motion from the workpiece and/or at the start of the feed a radial immersion motion into the workpiece.
Regardless of whether the gear skiving tool known according to the prior art has as the tool a cylindrical or a conical contour, fundamentally the same rolling motions are obtained in the metal-cutting process, i.e. the tool operates with and without a face offset. However, due to the path motion of the tool relative to the workpiece, at each moment of the engagement other clearance angles and rake angles are formed. In the most unfavorable case, during the cutter engagement rake angles of −50° or more can be formed, as a result of which the machining forces rise strongly, which ultimately, given inevitably arising vibration motions, can lead to not inconsiderable production inaccuracies. If the path motion is viewed in the reference system of the workpiece, then each reference point of the cutter moves on a three-dimensional cycloid. If the crossed-axes angle is neglected or an angle value of 0° is assumed, the trajectories in the external machining of a workpiece are epicycloids, and in the internal machining hypocycloids. The transmission ratio between the tool and the workpiece is decisively above the number of rollovers of the tool until the same point is reached after a 360° passage.
In order to prevent one or more defective teeth of the tool leading to corresponding defects in the finished workpiece, the number of teeth of the tool is chosen such that the number of teeth of the workpiece is a non-integral multiple. In the case of a non-integral multiple, the situation would namely arise that the tool, as it circulates, always machines the same tooth space with the same tooth, so that geometric abnormalities of a “cutting tooth” of the tool cause corresponding workpiece defects. Thus, a transmission ratio without a common denominator or with a prime number is preferably chosen, i.e. for example from 100 teeth of the workpiece to 29 teeth of the tool.
Given a positive crossed-axes angle, from flat cycloids evolve spatial roulettes, which can be used to analyze the motional paths of the faces of the tool. The kinematics of gear skiving turns out to be a complex motion in which each cutter of a tooth of the tool immerses successively into a tooth space of the workpiece and continues this radial motion in a rolling-down fashion as far as the tooth bottom, after which the tooth cutter on the opposite wall of the tooth space is moved back out. During the immersion and the withdrawal, the tool tooth cutter moves axially along the workpiece tooth width. The rake angle changes constantly and can even assume high negative values of up to −50°. At such high negative rake angles, the tools are placed under extreme load by the increasing cutting forces, which can give rise to considerable tool wear. Although the tools can be reground or exchanged for new tools, such works lead to downtimes in the production, which minimize the effectiveness of the process. In the case of conical tools, there is the added factor that the number of regrinding possibilities is limited due to the cone.